1. Field of the Invention
The present invention relates to a vibration type force detector, and more particularly to a force detector for detecting force through measuring the natural vibration frequency of a string loaded with a force in the direction of pulling the string.
2. Prior Art
It is well known that the natural vibration frequency of a string stretched with both the ends fixed depends on the tension and linear density of the string. This phenomenon can be used for the detection and measurement of force by making a string vibrate with its tension given by an unknown force to be measured. The natural vibration frequency of the string is proportional to the root of the tension, which is directly proportional to the force to be measured. The force can therefore be obtained as a value proportional to the square of the vibration frequency.
Vibration type force detectors devised in accordance with the above principle have been manufactured and used widely. However, the conventional detectors of this type have an important disadvantage that they can not be applied to a precision measurement of force or weight, because the string takes a long time to come to vibrate precisely at its intrinsic natural frequency after having been loaded with a force or weight to be measured.
In general, an exact response of the vibration frequency to a force or weight takes 3 to 30 minutes, depending on the material used for the vibrating string and on the method of fixing the ends of the string. This slow response causes such undesirable results as a creep, hysterisis and zero-point drift of a value to be measured. Therefore, the accuracy of conventional vibration type force detectors remains at relatively low values of 1/5000 to 1/20000.